The invention concerns hydraulic systems in which a reservoir serves more than one subsystem, and in which each subsystem includes means such as a pump for withdrawing its own supply of fluid from the reservoir. The invention is particularly applicable to an off-the-road machine such as a combine harvester.
The "dwell" time of returned fluid in the reservoir of a hydraulic system is time for de-aeration of the fluid, for cooling, and for settling out of contaminants. Typically, among several subsystems served by one reservoir, the tolerance level for adverse fluid condition factors such as entrained air, fluid temperature variations, contaminants and lack of positive head, varies according to the duty or function of the subsystem. If a simple reservoir is used, with little attempt to differentiate the withdrawal of fluid among the various systems, then the reservoir must have sufficient volumetric capacity, preferably with some reserve, to provide fluid in a condition compatible with the most sensitive or least tolerant subsystem. This requires one or more comparatively large capacity reservoirs, with the attendant disadvantages of comparatively high bulk, weight and cost. This solution is unattractive for mobile equipment, especially self-propelled machines with hydrostatic transmission and a variety of powered elements, such as combine harvesters in the agricultural field or, say, elevating scrapers in the construction equipment field. In these fields, control of overall machine size and weight are prime design requirements and systems permitting smaller reservoirs of higher power/volume ratio can make an important contribution. (The ratio is that of the total hydraulic power of a system to the volumetric capacity of the reservoir which serves it.)
The power/volume ratio of hydraulic reservoirs may be improved somewhat by such well known measures as the use of diffusers at return inlets and internal baffling-see, for example, U.S. Pat. No. 3,993,094, Spooner. But the known methods are basically passive and the requirements of the most sensitive subsystem still have a disproportionate influence on reservoir size so that possible gains are relatively limited.
To reduce problems arising from excessive turbulence common at the return inlet of a conventional reservoir, Brackin (U.S. Pat. No. 3,002,355) suggests, for a reservoir serving a single hydraulic circuit consisting of one pump and related actuator, an internal substantially closed passage between outlet and inlet. However, this arrangement essentially recirculates fluid without enhancement of fluid condition and offers little more than a surge tank function.
Kime (U.S. Pat. No. 4,371,318) attacks the pump cavitation problem by providing a "supplemental pressurization system", including a hydraulic fluid accumulator in the fluid supply line on the inlet side of the pump. But again, this is a single circuit hydraulic system and addresses only the cavitation problem.
None of the known hydraulic reservoir arrangements are especially adapted to serving a hydraulic system having two or more subsystems or to taking advantage of the opportunities in fluid management which such systems offer.